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May 19, 2018 at 8:04 comment added Peter Michor Using a resolvent integral along a simple closed curve containing some eigenvalues in the interior you can reduce the question to the finite dimensional matrix case. So Igor Khavkine's comment precisely answers your question.
S May 19, 2018 at 6:56 history suggested CommunityBot CC BY-SA 4.0
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S May 19, 2018 at 6:56
May 18, 2018 at 7:32 answer added Guest timeline score: 1
May 18, 2018 at 5:36 comment added Igor Khavkine If you replace $\Delta$ by some finite dimensional matrix, not necessarily symmetric/hermitian, then higher order poles of $A(z)^{-1}$ correspond to higher order Jordan blocks in the canonical form of $\Delta$. If you do the calculation explicitly for $\Delta$ being a single Jordan block, you'll see where the higher order singularities appear.
May 17, 2018 at 23:05 comment added Christian Remling $\langle f, (A-z)^{-1} f\rangle$ has positive imaginary part in the upper half plane for any self-adjoint $A$, so possible real poles must be of order $1$.
May 17, 2018 at 19:21 history edited Slm2004 CC BY-SA 4.0
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May 17, 2018 at 18:36 history asked Slm2004 CC BY-SA 4.0